Many ftp sites, especially that have anonymous access rarely being indexed by google. It's kind of old fashioned mode of filesharing which have minimal role for commercial purpose. A true direct mode... not the waiting link (cough.. direct link..) we seen today, says rapidshit and friends
Recently I find ftp search engine that still survived: http://proisk.ru (other is gegereka.com), a slow but extensive (probably excessive) ftp search engine. This is really great deal alternative that offer direct as well as parallel and resumable download. The sites itself is not compatible with Opera or at least I found its best for Firefox.
The accuracy is well said (with some redundant result). But sometime dead (red) ftp may be reported as alive (green) on the other side revived site still reported as dead. This meant indexes are quite not updated. One of good bet to tell the ftp is longlived is by its address. When it use nameserver not just IP or there is indication for static IP then the ftp may considered as reliable.
I have used with proisk to find many interested material (underground) that simply not indexed by google. While most are russian ftp, content is not always russian ocassionally the files just named in russian. So to improve accuracy you may need to translate your search string into russian language first using google translate.
Some ftp will try to counter your excessive download by blocking your IP (not quite sure about it) or by changing their IP to evade from you...
Nonetheless many ftp have many stuff that you rarely found through google yet for some people (like me) they are much better than current modern filesharing :)
To do quick search, just prepend your keywords (separated by dot) ie. http://discovery.channel.proisk.ru to find file related to discovery channel.